Many 401k plans have guaranteed income funds usually offered by an insurance company. That can be a safe place to park your funds if you are convinced a major decline is coming.

Check your plan rules. Some allow frequent transfers but others limit the number of transfers per year. As short bond funds and guaranteed income funds often pay very low interest these days, you don't want more there than you need and not for any longer than necessary.

Hence, its important to be right when you make that decision. If you are right, you can later buy equity funds at lower prices. If you are wrong you lose the gains you would have gotten in equities and get little reward from your fixed income investments.